1. Field of the Invention
This invention relates to a process for quickly and reproducibly obtaining .alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter referred to as .alpha.-APM) having a controlled water content.
2. Discussion of the Background
.alpha.-APM is used as a low calorie dietary sweetener which is about 200 times as sweet as sugar. The final product of .alpha.-APM is generally produced in the form of dried powder or granules. Processes for producing granules include a method wherein wet extrusion granulation is carried out before drying (Japanese Patent Application Laid-open No. 95862-1984) and a method wherein dry compression granulation is carried out after drying (Japanese Patent Publication No. 15268-1989). Various means for drying, such as flush drying, fluidized drying, and drying under reduced pressure may be employed.
The water content of .alpha.-APM after drying is desirably not more than 5 wt %, based on the weight of wet material, to minimize adhesion of material to the apparatus or walls of the pipe during subsequent transporting and packaging steps. However, the particular value of water content may vary considerably depending on the means and conditions of drying. Drying steps conducted under the same drying conditions often provide dried products with different water contents. For example, when the wet crystal cake obtained by crystallization is crushed and subjected to fluidized drying, it is inevitable that the wet crystal cake is unevenly crushed and unevenly dried depending on the unevenness of crushing, providing a variation of water content of about 1-2% in the final dried product. To avoid such problems, multi-step crushing and grinding of crystals may be conducted as the drying degree proceeds, so that the crystals are uniformly dried to have a water content around the desired value. However, determining the correct conditions and conducting such multi-step operations are often very complicated.
Moreover, the water content of the dried product is known to vary considerably depending on the heat load, i.e., the amount of wet crystals to be dried. For example, we found that under certain conditions wet crystals of .alpha.-APM with a water content of 25% could be continuously flush dried using hot air at 170.degree. C. to produce material having 3.5% water. However, if the amount of supplied wet crystals was reduced to half for convenience during the preceding step, the water content of the dried product was significantly reduced, i.e., from about 3.5 to about 1.6%.
Such variations become a problem particularly when the acceptable water content of the final dried product is fixed within a limited range. For example, in the above flush drying process, the dried product having only 1.6% water content would have to be redissolved, and again subjected to the steps from recrystallizing to drying, if the standard water content is restricted to 3-4%. Such problems mean that operations beyond those which are strictly essential for production must be performed and equipment availability is reduced, which are two of the main reasons for increased cost of production. To avoid these variations in water content the drying conditions, including the amount and temperature of hot air, should be changed in accordance with the load. However, it requires much labor to determine the correct drying conditions.
When the acceptable water content in the dried product is fixed to a defined value or lower, manufactures earnestly try not to produce products which are above the standard. In practice, that means that they frequently employ too much drying strength during the drying step. As a result, overdried products with relatively low and uneven water content are often obtained. Considering the stability of the crystals and DKP production during storage of the dried product, in addition to adhesion of the dried product in the steps after drying, water content of the dried .alpha.-APM is desirably within the range of 2.5-4%. It is not desirable to evenly obtain overdried product with relatively low water content. Further, .alpha.-APM tends to produce IIB-type crystals with poor solubility upon overdrying. Accordingly, to avoid losses due to production of products below the standard water content, and to further enhance ease of handling and storage stability as well as solubility of the dried product, it is important to control the water content of the dried .alpha.-APM to a fixed value.